1. FIELD OF THE INVENTION
The present invention relates to a method for surface treatment for easily forming a layer of a nitride or carbonitride which does not substantially contain iron nitride: (referred to as "Fe--N" hereinafter), at a temperature of not higher than 700.degree. C. on the surface of metal materials which are used for dies, jigs, machine parts and the like.
2. DESCRIPTION OF THE RELATED ART
It is known that metal materials (referred to as "materials to be treated" hereinafter) are improved in wear resistance, seizure resistance, oxidation resistance, corrosion resistance, etc. when they are coated with a surface layer of a nitride or carbonitride such as chromium (Cr) and titanium (Ti). Several methods have been proposed for forming the surface layer. One of them is a method of coating a material to be treated with a surface layer of a nitride or carbonitride of Cr, vanadium (V), Ti, etc., by the aid of plasma CVD (chemical vapour deposition) using a halide of Cr, V, Ti or the like (For example, Japanese Patent Laid-open Nos. 65357/1980 and 164072/1980). This method enables a surface layer to be formed without causing a heat-induced strain to the base metal of the materials to be treated because the treatment is carried out in the temperature range of lower than 700.degree. C. , which is the Ac, transformation point of iron. This method, however, is disadvantageous in that it is difficult to form a surface layer excellent in throwing power of electrolytic colouring: and adhesion. It is also disadvantageous in that the treatment requires a high-purity and expensive metal halide, such as titanium tetrachloride (TICl.sub.4), as a constituent of the surface layer. Since such a metal halide has a sufficiently strong reactivity to corrode a reactor or pipes, and is also toxic, it is difficult to handle. The treatment therefore requires a complicated process and an expensive apparatus. In addition, since it is necessary to carry out the treatment in hydrogen or under a reduced pressure, the method is poor in efficiency.
In the ordinary heat CVD without using plasma, a low-temperature CVD for forming a coating layer of a carbide or nitride of tungsten (W) at a temperature of 350.degree. to 700.degree. C. has been proposed. (N. J. Archer (Wear) 48, 1979, P. 237)
In this method, tungsten hexafluoride (WF.sub.6), hydrogen (N.sub.2), hydrocarbon, and argon (Ar) are introduced into a reactor and reacted at the above-described temperature so as to form tungsten carbide (W.sub.3 C, W.sub.2 C) on the surface of the material to be treated, or WF.sub.6, H.sub.2, ammonia (NH.sub.3), and Ar are reacted so as to form tungsten nitride (W.sub.2 N) on the surface of the material to be treated. In this method, WF.sub.6 used is difficult to handle because of its corrosiveness and toxicity. Since H.sub.2 is used in this method, the furnace cannot be freely opened or closed so that it is impossible to insert and take out the material to be treated with efficiency. Furthermore, no industrially useful coating layer has been obtained by using-elements other than W.
On the other hand, a surface treatment by CVD using a fluidized bed furnace has, been proposed (Japanese Patent Laid-open No. 243178/1980 corresponding to U.S. Pat. No. 4,623,400). This proposal is quite different from the present invention in structure and effect. In the former case, inert particles (particles of alumina or the like which do not participate directly in a reaction for forming a surface layer) are used for a fluidized layer (also called a fluidized bed) and a metal halide is used for the material forming the surface layer. The metal halide is mixed with an inert, gas, introduced into the fluidized bed from the lower part of a dispersion plate (also called a metal diffuser plate) which is placed directly under the fluidized bed, and mixed and reacted with a reacting gas, such as ammonia supplied directly into the fluidized bed without passing through the diffuser plate, thereby forming a nitride or the like on the surface of the material to be treated which is buried in the fluidized bed.
In this method, the fluidized bed itself is inert and it has no direct chemical act on the reaction for forming the surface layer. The fluidized bed also is neutral and it has a role as a mere heating medium. The reaction itself is therefore substantially the same as the known CVD reaction, both reactions having the same characteristic features.
Since the metal halide is introduced into the fluidized bed from the lower part of the diffuser plate, immediately after the gas of the metal halide enters the fluidized bed, which is above the diffuser plate, the metal halide starts reacting with the reacting gas, and a nitride or the like is deposited on the surface of the inert particles which constitute the fluidized bed. This causes a large consumption of the metal halide and a lack of the gas of the metal halide which should reach the surface of the material being treated. When a nitride or the like is formed on the surface of the inert particles, the adhesion of each inert particle increases and the fluidity is prone to be lowered. Furthermore, an extreme variation in the gas concentration of the metal halide is caused from the lower part to the upper part of the fluidized bed and it is difficult to obtain the uniformity of layer thickness in the fluidized bed. In addition, since the metal halide, which has strong corrosiveness passes through the diffuser plate, the diffuser plate is corroded and the life thereof is shortened. Even an adoption of a method for preventing the corrosion wherein the metal halide is directly introduced into the fluidized bed through a pipe or the like, instead of the diffuser plate, in the same way as a reacting gas, such as NH.sub.3, makes it more difficult to obtain the uniformity of the layer thickness in the fluidized bed.
Accordingly, the above-described proposal which uses inert particles and a metal halide (Japanese Patent Laid-open No. 243178/1986 corresponding to U.S. Pat. No. 4,623,400) cannot be an industrially practical method for forming a surface layer.
In order to overcome the disadvantages of the conventional methods for surface treatment, the present inventors achieved an invention relating to surface treatment which is characterized by forming a surface layer of a nitride or carbonitride: of Cr, V, Ti or the like on the surface of an iron or an iron alloy material, which is a material to be treated, at a temperature of not higher than 650.degree. C. (Japanese Patent Laid-open No. 128164/1988 corresponding to U.S. Pat. No. 4,871,401). According to this method, a material to be treated is inserted into a treating agent composed of a refractory powder of alumina or the like, a material containing Cr, V, or Ti, and a halogenated ammonium salt and/or a metal halide, which is fluidized by a nitrogen (N) -containing gas or the like, and heat-treated at a temperature of not higher than 650.degree. C., so as to form a surface layer composed of a nitride or carbonitride of Cr, V, or Ti and Fe on the surface of the material to be treated. This method, however, is disadvantageous in that if the amount of nitrogen is large, the rapid nitrification forms a fragile Fe--N layer or a diffusion layer at the lower portion of the nitride layer, which greatly lowers the toughness of the base metal. It is also disadvantageous in that since a large amount of Fe--N exists in the layer, the layer is poor in hardness and inferior in wear resistance, corrosion resistance and the like. Development of a treatment for forming a nitride which does not substantially contain Fe--N by controlling the nitrification reaction has therefore been demanded, but it is impossible to form a nitride layer having a thickness suitable for practical use and not substantially containing Fe--N at a temperature as low as not higher than 700.degree. C.